This invention relates in general to electroforming and more specifically, to an electroforming mandrel and method of fabricating and using same.
Prior art mandrels utilized for electroforming operations are often plated with a metal to improve the durability of the mandrel and to facilitate removal of the electroformed article. These electroforming mandrels usually have straight parallel sides to facilitate removal of the electroformed article from the mandrel. A slight taper may be imparted to the mandrel sides in the direction of removal to further aid in the removal of the electroformed article. It is essential that the circumference of the sides along the axial length of the mandrel remain the same or decrease in size so that the electroformed article can be removed from the mandrel without damaging the electroformed article or the mandrel.
Electroforming mandrels of the prior art are usually coated with a protective metal layer to enhance durability and to facilitate removal of electroformed articles. When cylindrical mandrels having flat ends (e.g. ends in a plane that form a right-angle with the parallel sides) are plated by electroplating techniques, an edge effect is encountered due to electric current distribution characteristics. This edge effect results in thicker deposits at the ends of the parallel sides adjacent to the flat mandrel ends. These thicker deposits cause a plated mandrel to have a larger circumference at each mandrel end thereby preventing removal of the electroformed article from either end.
In order to prevent thick end deposits during plating of mandrels having flat ends, a disk shaped electrically conductive "robber" may be secured to each flat end of a mandrel. This arrangement allows the thicker deposits of plating material to form at the flat ends of the robbers rather than at the flat ends of the mandrel. After plating, the robbers are removed and the plated mandrel is used for electroforming. Unfortunately, the ends of this type of mandrel are not protected by any plating and, therefore, tend to corrode during use. End caps may be secured to the ends of the mandrel prior to electroforming to protect the unplated surfaces from corrosion. However, the intersection between the end caps and the plated surface of the mandrel is still susceptible to corrosion and causes a build up of deposits which resemble coral. Moreover, the electroformed material tends to form a deposit in the crevasse between the end caps and the ends of the mandrel core. To avoid these undesirable effects, a ring shaped shield may be applied to cover the intersection between the end cap and the end of the mandrel. Such shield must be applied to the mandrel prior to electroforming and must be removed subsequent to electroforming so that the electroformed article can be removed from the mandrel. These operations increase the number of handling operations per electroforming cycle and increases the likelihood that the outer surface of the electroformed article will be contaminated during handling by foreign materials such as finger prints.
Other techniques used to control the plated coating thickness include the use of shades and/or varying porosity screens which can be positioned within the bath to minimize the end effects. These techniques, however, require adjustments to accommodate any change in mandrel dimensions and/or changes in operating parameters. The electric current distribution pattern is so dependent on operating conditions that adjustment of shading during operation is necessary to compensate for normal changes in operating parameters during plating (e.g. temperature increases) to achieve the same results achieved with robbers. Learning how to move and position the shades for any given process requires exhaustive testing.
The many operations required for applying, adjusting and removing masks and bottom protectors increases the time and handling requirements and thwart conversion to rapid, automated processes that utilize, for example, automatic electroform parting techniques.